user: Remove use of Message:text() from User::isUsableName()

user: Remove use of Message:text() from User::isUsableName()

None of the current translations use of "&", "{" or "[", which
presumably means they don't require tranformation. I'd also
very much prefer such dynamic constructs not be added into
this system which would make it rather unpredictable given these
reserved usernames shouldn't change from time to time.

The only match for the below command is qqq.json,

> $ git grep -E '"(double-redirect-fixer|usermessage-editor|\
> proxyblocker|sorbs|spambot.username|autochange-username)"\
> ' | grep -E '[{&[]'

This code was originally introduced with r37928 (5ad5cb4f0a64),
which used wfMsgForContent() and made no explicit mention of
transformation, it just happened to be the default. Later, when
that function was deprecated, it got batch-replaced by
->inContentLanguage()->text().

Bug: T189966
Change-Id: Ia4ddf215e83f15de552f8311b9e737559c72b49a